The present invention relates to an automatic transmission control system for motor vehicles. More particularly, the invention relates to an automatic transmission control system which can minimize a gear shift shock caused by an accelerator work during the gear shift, without the complexity of the gear shifting operation.
In recent days, a motor car with the automatic transmission for automatically shifting gears depending on a vehicle speed (the rotation speed of the output shaft of the automatic transmission) and the quantity of depression (throttle opening) of the accelerator pedal, has come into common use.
FIG. 11 is a block diagram showing the construction of a conventional automatic transmission control system.
In the figure, reference numeral 1 designates an accelerator pedal; 2, an accelerator opening sensor for detecting a depression amount of the accelerator pedal 1; 3, an accelerator cable, connected to the accelerator pedal 1, for operating a throttle valve; 4, a throttle valve, mounted within an intake pipe 6 connecting to an engine 5, for controlling the quantity of intake air; and 7, a throttle opening sensor for detecting an opening degree of the throttle valve. An automatic transmission 8 is connected to an output-shaft rotation speed detecting means 9 for detecting the rotation speed (corresponding to a vehicle speed) of the output shaft. An input-shaft rotation speed detecting means 10 is used for detecting the rotation speed of the input shaft of the automatic transmission 8. A control unit 11 for controlling the automatic transmission 8 receives signals representative of the rotation speed of the output shaft, the accelerator opening, throttle opening and the like from the output-shaft rotation speed detecting means 9. Reference numeral 12 designates a hydraulic controller for the automatic transmission 8, and 13 is an electromagnetic valve included in the hydraulic controller 12.
Reference numeral 14 designates an injection valve for injecting fuel to the engine 5. A by-pass air path 15 turns aside from the throttle valve 4. A by-pass valve 16 is for controlling the cross sectional area of the by-pass air path 15.
The injection valve 14, the by-pass valve 16 and the like are controlled by the engine electronic control unit 17. The engine electronic control unit 17 receives data signals indicative of temperature of engine cooling water, the quantity of intake air to the engine 5, engine speed, atmospheric pressure, temperature of intake air, and the like, and controls the quantity of injection fuel, ignition timing, by-pass valve timing, and the like. The engine electronic control unit 17 and the transmission control unit 11 are interconnected by means of a transmission line 18 through which data is transferred between them.
A first example of the automatic transmission control using the conventional automatic transmission control system will be described.
A driver steps on the accelerator pedal 1 in accordance with driving conditions. The accelerator pedal 1 and the throttle valve 4 are connected to each other by the accelerator cable 3. The opening of the throttle valve 4 changes in accordance with the quantity of the depression of the accelerator pedal 1.
The transmission electronic control unit 11 makes a decision on the gear change in accordance with the sensor information of the accelerator opening sensor 2 or the throttle opening sensor 7 and the output-shaft rotation speed detecting means, and sends a gear change instruction signal to the automatic transmission 8.
FIG. 12 is a graph showing a variation of the throttle opening and the accelerator opening with respect to time t including the gear shift duration of time.
In the figure, the gear shift duration ranges from a time point A to another time point B. The graph shows a state in which the accelerator pedal is depressed during the gear shift, and as a result, the throttle opening 71 is varied.
A second example of the automatic transmission control using a conventional automatic transmission control system will be described.
Description of the second example is limited to the automatic gear shift control in the down-shift from high to low gear, for ease of explanation.
In the down-shift mode, the automatic transmission control system selectively uses the transmission control depending on a state of the engine, a power-on state or a power-off state.
Here, the term "power-on" means a state in which the engine receives a positive power (for example, when the accelerator pedal is depressed). The term "power-off" means a state in which the engine receives a negative power (for example, when the accelerator pedal is not depressed).
In the power-on state of the engine 5, if the friction engagement elements (e.g., the clutch, brake, and the like of the automatic transmission) engaged into the high gear are disengaged, the rotation speed of the input shaft increases and the input shaft reaches by itself the rotation speed attained after the gear shift. Accordingly, the friction engagement elements are made to engage into the low gear when a detected value detected by input-shaft rotation speed detecting means 10 is equal to that by the output-shaft rotation speed detecting means 9.
In the power-off state of the engine 5, the friction engagement elements engaged into the low gear are disengaged in a manner that the friction engagement elements engaging into the high gear are disengaged to increase the rotation speed of the input shaft of the automatic transmission 8 by the friction engagement elements engaging into the low gear, and when the detected values of the input-shaft and output-shaft rotation speed detecting means 10 and 9 are equal to each other.
When the shift-down from high to low gear in the state of the power-off of the engine 5 is done under the conditions that the vehicle speed is low and the throttle opening is small (immediately before the vehicle stops), it is difficult to minutely control the torque capacity of the friction engagement elements since the engine braking force is small. As a result, a gear shift shock occurs and gear shift time is too short.
Before the down-shift starts immediately before the vehicle stops, if the engine is in the power-on state in the vicinity of the boundary between the power-on and the power off states (FIG. 13), the gear shift proceeds and if the engine speed increases, the engine state frequently changes to the power-off state during the gear shift. This causes a gear-shift shock, called "bumpy motion" of the drive system.
To cope with this problem, Japanese Unexamined Patent Publication (Kokai) Hei-4-91332 discloses a unique technique.
The construction of the automatic transmission control system and its peripheral devices of this publication are basically the same as that shown in FIG. 11. Accordingly, the automatic transmission control for the down-shift of the automatic transmission 8 will be described using the automatic transmission control system of the publication. The description will be given with reference to FIG. 14.
To start with, in a step S1, it is determined whether or not a vehicle speed detected by the output-shaft rotation speed detecting means 9 is equal to or lower than a preset value V0 and a throttle opening detected by the throttle opening sensor 7 is equal to or smaller than a preset value TH0. In this instance, the preset values V0 and TH0 are those values obtained immediately before the vehicle stops.
If the answer to the step S1 is YES, this state of the transmission is recognized as the down-shift immediately before the vehicle stops, and a flag is set to "1".
In a step S3, the hydraulic circuit of the automatic transmission 8 is altered in accordance with the down-shift from high to low gear, independently of the recognition result in the step S1.
In a step S4, it is determined whether or not the flag was set to "1" in the step S2. If it was set to "1", the current state of the automatic transmission is the down-shift just prior to the vehicle stop. Accordingly, an idle-up instruction is issued in a step S5. In response to the instruction, the opening of the by-pass valve 16 is increased to cause the quantity of air flowing through the by-pass air path 15 to increase. The engine is placed to the power-on state.
In a step S6, hydraulic control is done in order to effect the shift-down. Specifically, a duty ratio of the electromagnetic valve 13 of the hydraulic controller 12 is controlled. As the result of the hydraulic control, the friction engagement elements of the automatic transmission 8 is changed from the high gear to the low gear.
In a step S7, it is detected whether or not the input shaft rotation is synchronized with the output shaft rotation by using the input- and output-shaft rotation speed detecting means 10 and 9. If both rotations are synchronized with each other, the control recognizes that the gear shifting operation is completed. In a step S8, it is checked whether the flag is set to "1". If it is "1", the idle-up operation is stopped. The opening of the by-pass valve 16 is reduced to the original one, so that the quantity of the air flowing through the by-pass air path 15 is reduced. In a step S10, the control stops the hydraulic control operation, and in a step S11 resets the flag to "0".
Variations of various parameters in the automatic transmission control as described above are illustrated in (a) to (f) in FIG. 15.
In FIG. 15, (a) shows a variation of the rotation speed of the input shaft; (b), an output shaft torque; (c), an opening of the by-pass valve 16; (d), an opening of the throttle valve 4; (e), the quantity of intake air; and (f), a duty ratio of the electromagnetic valve 13. The axis of abscissa represents time.
In the figure, A represents a gear shift start point, B is a point where the idle-up instruction is issued, and C is a point where the input shaft and the output shaft are synchronized in rotation and also the idle-up instruction end.
In the conventional art, when the gear shift is carried out, the system control recognizes the state of the automatic transmission as the down-shift just prior to the vehicle stop, and the system control issues an idle-up instruction. The engine maintains the power-on state unless the gear shifting operation abnormally proceeds.
Accordingly, in this conventional art, even in the state of the down-shift just prior to the vehicle stop, no gear-shift shock takes place.
The conventional automatic transmission control systems thus constructed and operated, however, have the following problems.
In the first conventional art, even when the accelerator pedal is operated during the gear shift, the throttle opening varies. Accordingly, a degree of the gear shift is altered, even during the gear shift, in accordance with a change of the throttle opening. This alteration makes the gear shift control complicated.
Further, the throttle opening changes depending on the acceleration opening. As a result, output power of the engine changes to cause a gear shift shock.
In the second conventional art (described in Japanese Kokai-Hei-4-91332), the vehicle speed and the throttle opening are smaller than preset values. Even if the quantity of the intake air is increased by the idle-up, the engine is in the power-on state in the vicinity of the boundary between the power-on and power-off states before the gear shift starts. When the throttle opening, as shown in (d) of FIG. 15, is further decreased during the gear shift, the quantity of the intake air to the engine (e) of FIG. 15 also decreases, so that the engine is placed to the power-off state. In this case, the torque of the output shaft of the automatic transmission is varied as indicated by a broken line, not a solid line, in (b) of FIG. 15. This appears as a gear shift shock in the drive system.
In the second conventional art, the idle-up instruction is issued during the gear shift. A time elapses from an instant that the quantity of the intake air increases till the engine output increases. The gear shift proceeds during the course of increasing the engine output to a sufficient value. When the engine is in the power-on state in the vicinity of the power on/off boundary before the gear shift starts, the output shaft torque of the automatic transmission varies as indicated by the broken line in (b) of FIG. 15, causing a gear shift shock.
Additionally, the general automatic transmission control system selects a suitable gear shift control mode depending on a state of the engine. Where a state of the engine changes during the gear shift, a complicated gear shift control is required.